Crystal contacts of which one element is mainly silicon



Oct. 2, 1951 JONES ET AL 2,569,892

CRYSTAL CONTACTS OF WHICH ONE ELEMENT IS MAINLY SILICON Original Filed Aug. 10, 1942 FIG.|

uuveu-rotzs: DOUGLAS ENFIELD JQNES HARLE$ EEl RANSLEY JOHN wAnen RYDE STANLEY VAUGHAN WILLIAMSY DE EASED;

BY FRANCI MABEL WILIJAMS,

EXECUTRIX ATTORN Patented Oct. 2, 1951 CRSTAL CONTACTS F WHICH oNE ELEMENT IS MAINLY SILICON Douglas 'En field Jones, Harrow, Charles Eric Ransley, Sudbury, and John Walter Ryde,

London, England,

and Stanley Vaughan Williams, deceased, late of Kenton, England, by Frances Mabel Williams, executrix, Kenton, England, assignors, by mesne assignments, to Haz'eltine Research, Inc., Chicago, Il1., a corpo- 'Origina'l application August 10, 1942, Serial No. T454390. Divided and this application March "20, 1947,'Serial No. 736,094. In Great Britain "August "20, 1941 16 Claims.

invention relates to :silicon crysta'l elements for wave=signal translating devices of the type comprising a semiconducting crystaland 'a metal (usually a "metal .point) in'con-tact With it. Such contacts can be used as rectifiers of altermatin current or as mixers, that is to say as nonlinear impedances to which are applied an oscillationof frequency $1 and another oscilla- *-tion* of frequen'c'y fz, so that oscillations are produced ha'vingiirequencies tpjiiqfz) where p and -q are integers.

The present application "is a division of application Serial No. -4'54;'290, -filed August 10, 1942, now Patent 23419,561, graritedApril "29, 1947, and entitled Crystar-Gonta'cts of Which One #Elemen't lslillai'rily -silition'. I

Objects of the inv en tion are to provide improved silicon-crystal contactelements"andwavesignal translating devices =-embodying such crystals thatare more uniformor more .efiicient or both than those of the said type known hitherto. The uniformity aimed at means that the electrical characteristic of the contact should be as nearly as possible independent 'of 'the .position on the'crystal of a metal p'oint forming the other element' of the contact. Efficiency generally requires a high rati'o of impedance in one direction (reverse impeda'n'ce) to impedance in the other "direction tforward im'pedance). It may also require that the absolute values of the forward impeda'n'ce should be high *"or, alternatively, that they should be low. It' may-also require that the forward direction should be that "in "which positive ch'arge "flows :from the crystal to the metal -(po'sitive contact) or that it "should be that in which negative charge-flows ifrom the crystal to the metal (negative contact').

When theeontact is-used as a mixer, efficiency requires that the sign-al-to-noise *ratio should be relatively high. Efii'ciency also :requires that the performances! the contact judged by any of the ffore'going criteria should deteriorate as little as possible with use or with age.

"In accordance with :the :present invention, a

silicon-crystal elementifor a wave-signal translating device comprises as'oli'd solution of silicon substantially free rfrom impurities usually present in commercial=siliconiand:a predetermined small amount of 'at least *one of the two metals alu- Ininium fand beryllium, the ratio of the number of -atom s-of metal to the number of atoms of the silicon the product having a value between limitsof theorderof Awand /50. v

-siliconz-is one of the semiconductors most widely-lused in-=crystal-conta0ts We have tound that con at present available, the introduction of the selected "additive metal must be preceded by a purification. This purification can be'eifected by subjecting commercial silicon to =theprocess described by N. P. Tucker in the Journal of the Iron and Steel Institute, vol. CXV(1), p. 412, 1-927. This process removes only those impurities that'are not contained in the silicon crystals, but occuras an additional phase, usually in thevinterstices between the silicon "crystals. Commercial siliconis usually prepared by=ceoling=a melt slowly; then practically all the deleterious impurities are in the interstices and can be removed by Tuckers process. Ifthat process fails Withany sample of silicon -it is probably because the silicon has been quenched rapidly; then the silicon should be -re-melted and cooled slowly before it is subjected to the Tucker process. I

The additive metal is preferably introduced by melting a mixture of the pure silicon and the additive. During this process care must be taken not to introduce undesired impurities. Those that might be derived from the air (a small quantity of oxygen-is not necessarily deleterious) can be avoidedbv melting in a suitable atmosphere; the most suitable we have found is a vacuum, that is to say residual gas at apressure of not more than 0.001 mm. of mercury. But itiis difficult, or perhaps impossible, to find any crucible to contain the melt that does not react slightly with the melted silicon. This difficulty can be overcome by using a crucible consisting of -or lined with pure beryllia; for then the only impurity introduced from the crucible is beryllium, which is not harmful but beneficial. It might Ice-possible to introduce from the crucible all the additive metal characteristic of the invention; but in view of .patent application Serial No. 446.310, filed June 8, 1942,no'w Patent 2,419,966, granted May. .6, 1947. we exclude from the scope of this inventionany process of manufacturing the mainly silicon element ot a crystal contact and often desirable.

3 which comprises only the steps mentioned in the claims of the specification of that application. If therefore the first step in the process consists in grinding relatively impure silicon to a fine powder and treating it with chemical'reagents so as to produce a relatively. pure product, the

process will not lie within the scope of the present invention unless part at least of the additive metal is derived from a source other than a con tainer in which the silicon is melted. But it is the best oualities, it is generally necessary and always desirable that, after the introduction of the additive, the crystals should be subiected to an appropriate treatment modifying their surface. One part of this process is a controlled oxidation; this part can be omitted only if the necessary oxidation has been effected during the melting or the cooling of the melt. We have occasionally succeeded in producing sat sfactory results by this method; but it is difficult to control, and we greatly prefer to remove any surface oxide from the coo ed melt with hydrofluoric acid and then to heat the roduct in a definitely oxidizing atmosphere, Further we pre er no t to attempt to produce the o timum surface oxidathe layer of oxide with hydrofluoric acid.

Other surface treatments are also permissible topolish the fragments of the melt, or at least thatpart of them which is to contact with the metal point, before the controlled oxidation. Again, crystals are often soldered into metal capsules; then it is desirable to plate with metal that part of the crystal that is to lie in the capsule,

in order that the solder may adhere to it.

One method of performing the invention will now be described by way of example, with reference to the accompanying drawing, in which Fig. 1 shows the completed crystal contact device; Fig. 2 shows a silicon fragment temporarily mounted in readiness for a step in the process of preparation; and Fig. 3 illustrates the step of mounting the same element in a capsule.

Referring to Fig. l, the completed crystal contact device consists of a ceramic tube In capped at its ends respectively by a metal capsule H and a bush 12. The crystal body i3 is set in the capsule II by means of a low-melting alloy 14.

One or more metallic contact elements I5 carried by a metal rod l6 co-operate with a prepared face 11 of the crystal body, the rod I6 being held fast in the bush |2.. V

The, starting material is commercial silicon which has been ground and passed through a 200 mesh sieve; the material contained the following metalsin the following amounts reckoned as oxides: (Al2O3+ZrOz) 1.01%, Fe2O3 0.96%,

TiO2 0.021%, MnO 0.08%, BaO 003%, Cao 1.13%,

Thus it is usually preferable 7 0.003%, CaO 0.008%, CuO 0.001%, M'g'o less? 0.03%, F6203 les than-0.005%,f 'rio2- as than 0,0005%, MnO less than 0.001%, BaO less than than 0.002%. In step (2) the purified material is melted in vacuo (pressure less than 0.001 mm.- of mercury) in a pure beryllia crucible, tdgetherf with aluminium infl'a n atomic proportion of 1 Alto 400 Si. The beryllia was fired to 1400" Q,- ground and washed with nitric acid before being made into a crucible.

In a further-step: (3) a fragment of the cooled melt has a plane part of its surface highly polished by any of the processes customary among metallurgists, .for example grinding with emery of increasing fineness and finally polishing with alumina or magnesia or both. In a further step (4) the whole surface of the fragment is then dipped for 5 minutes into commercially pure 40% hydrogen fluoride diluted with an equal quantity of water. The main purpose of this treatment with hydrofluoric acid is to. prepare theipolished surfacefor the subsequentjoxidation; accordingly it is not necessary, though it is usually .convenient, that the unpolished part' of the surface should be dipped. r Y 7 1 f In a further. step (5) the necessary surface oxidation is performed. For this purpose the body resulting from step (4) is placed-.ona "flat silica tray (together possibly with other bodies at the same stage of preparation); and introduced into a silica tube furnace filled with air at a region thereof maintainedat1050 C There is a definite optimum temperature for-this treat- 'ment; the temperature should be observed, erg.

by a thermocouple, and kept as near to 1050 C. as possible. The body is maintained. in'the said region for 2 hours and then allowed to 0001,. Art atmosphere of oxygen at acontrolled pressure may be substituted for the air in the furnace"i then the time of heating will depend on the pressure of the oxygen. w r

In a further step (6) the treatment "gi-venain step (4)is repeated. Now part of its object is to prepare the unpolished part for step (7); accordingly the unpolished part must be dipped, In the process now being described in detaif the clipping of the polishedpart' is equally necessary, inorderthat part of the oxide layer formed in step (5) should be removed; but, as explained heretofore, it might possibly not be in a modification of the process.'--After the dipping, the body is washed with water. 1

In a further-step (7) a metallic layer is deposited on some or all of the unpolished surfaces. The layer must not be deposited on the polished surface; accordingly as. shown in Fig. 2, this is first protected by coating the flat end of a rod IS with a layer of adhesive l9, which maybe the material known as monostyrene, and pressing the polished surface [1 of the body I3 against this layer. The exposed unpolished surface is then coated with copper by'a method described by Bedel in Comptes Rendus, vol. '192,(193l)-, page 802; It consists merely in dipping the surface for 5 to 10 seconds into a solution of cuprous oxide in 20% hydrofluoric acid; the copper layer deposited may be subsequently thick-- ened by electrolysis. Copper is not deposited on tungsten or molybdenum by' th'is'process; accordingly the rod 18 may be conveniently of tungsten or molybdenum. The coated'surface is then washed and dried. Thebody can then be mounted in the metal capsule II by means of solder or some other suitable low-melting al- 10y intervening between the metal layer and the 45 to *the :axis of ..the -:wire. "tween .thewire and xthextpolished crystalisurface .may vabe .l-.1.5 .gm. weight. .:If ithe c.ontact is :tested with :an applied D. C. E. ;-M. .of 0.7 v.volt, -:.then after little :searching, aifpoin'tfcanrbe .;found where the forward. impedance is about.:20.0

suggestion is practically useful.

capsule. Forrtlnis purpose :the rod til :isainserted in the tube Ill] :so that theicrystatenters the molten alloy M inzthelcapsule H, which i s-.:tem- .porari'ly placed on .aan endof therltube. After the .alloy has setytherod I18 is :detached from The mounted crystal body:isrthenreadyior .final assembly intozthe.structure-shownin Fig. .1.

A suitable .metal element .to :coeoperate with ya cr'ystalso preparedis the. end. of .a tungstentwire 01.2 :mm. in diameter, thisend being. sheared off along :the cleavage :plane .which :lies at .about The :pressure beohms, ..and theratio of reverse toforward impedanceisatileast .30. "With a lighterpressure ayratioof 10.0501 more can usually .be obtained,

.butthen the forward impedance is .greater. .The

.contact will usually gstand without appreciable .deterioration'thexpassage across it, foria period of 10 to 60 .seconds, 1of..a current (limiting current) of-$0130.80 ma. Comparable average figures for the commercialsilicon that was the startingmaterial, polished .as described herein- :before but .not treated otherwise, :are forward impedance 180 ohms, ratio ofrreverseto:for-ward impedance 5, limiting current 5-20 ma.

.If :a lower forward. impedance is :required, the pressure between crystal :and wire "may .be increased ,to 30-50 :gm. weight. .A forward impedancein the :neighborhoodrof 100 ohms can then .be obtained, but ..the average ratio of 're- .yerseto.forwardjmpedance isllikely to be -20,

and the limiting .:current 3 3.0160 ma.

If a;high limitingccurrent.isnotcso important ;as the impedance ..characteristic, :the aforesaid sheared tungsten point may :be :replaced :by .:a

dime tungsten point obtained ..by .the well-known :process of etching with Langmuiris solution.

' characteristic. The fimprovement is :usually' not so much in the impedance .asain the 'limiting current that the contact will stand without deterioration. Nonovelty is claimed for the bare suggestion that whiskers .on the same crystal may be connected in parallel; but, so:far.-as we are aware, .no disclosure has been made before how crystals might be prepared that are at once of high efficiency and also so uniform -thatthe While therehave been described what are at present l considered to be the preferred embodimerits of this invention, it will be obvious to i6 termined .ssmall iam-ount :of i at least tone of the :atwo :metals :aluminium :and .beryllium, the ratio 1 of ;the. number of atoms .of metaltto' the .number of atoms of said siliconiin the product having a .5 value of ..thetorderpf 11/ .2. silicon-crystal ielementfor a wave-signal a translating :device -.comprising, solid solution h of 1 silicon substantially .IfIBB ;from impurities .usually-.present .in commercial siliconcyet .having; im- 10 zpurities in the .form 10f .metallic oxides totaling iofiithezorderofs 0.05- per. cent. and a predetermined small amount .of 'atleast one of theltwo metals aluminium :and. beryllium, the ratio of the i number-10f atoms ofimetal ;to':the .number:.of atoms of 15 said ssilicon ain ..the :product .having a value between limits of the order. of 14 and 93 13. ..A silicon-crystal element fora wave-signal translating device comprising, :asolidsolution of silicon having 'a purity-of the. orderof 99.952136! ;cent. 3: and .a apredetermined.smallu amountiof at ,leastloneof the:two.metalsnaluminiumiand beryllium, tithe ratio :.of the :number of ..atoms of .metal .to the numberofaatoms of ;said.siliconlin the :product .having .;a value between limits .of the .order ;of: -and 4. 4 A zsiliconecrystal element for .a uwave-signal translatingidevice comprising, a solid solution of silicon substantially free. from impurities usually present-in commercial silicon and 1a predeter- -ao mined small amount of :at least one :.of l the two metals "aluminium-and beryllium, the ratio of the number of atoms of metal to the numberrofatoms of said silicon. in the :product having a value heitweenlimits of theorder of c 4 and :5. .-A silicon-crystal :element for a .waveesignal i translating device; comprising, ia solidlsolution of silicon of :a puritywin excess:of 99 per: centiandra predetermined .small ,amount :of at least one i of :the :two metals -.=aluminium and beryllium, ..the i0 -ratioof the :number of .'atoms;of .metal to the :number of atoms of'silicon inthezproduct having 'a value between limitsoftheorder. of and A ..6. IA silicon-crystal c element :fora wave-signal xtranslatingrdevice comprising, a. solid solution of silicon substantially free=fromimpurities usually present v;in commercial silicon and a predeter- ;minedsmall.=amount of at least one of the two .metalstaluminiumiand beryllium, .theratio of the "number of'atoms ofmetalatothenumber ofsatoms of said silicon initheproduct havinga value hettween' limits. of ,thecorder. of 4 and 4 said ele- ;men.t.having;ahydrofluoric acid treated surface adapted toxbe-contacted by a metallic element included said translating device.

"7. A-silicomc-rystal element" for a wave-signal translatingcclevice. comprising, a solid solution of silicon substantially free from impurities usually present in commercial siliconand a predeterzmined. small amount :of .at least one of the two metals aluminium and: beryllium, the ratio of 1 the :numberof: atoms ofimetalato'the number; of atoms ofisaid siliconinathe product having a value' be- .tween "limits of -ithe order 4 of $4 and 4,6 said .:element :having 12am 1 oxidized surface adapted to be contacted by a metallic contact element included in said translating device.

8. A silicon-crystal element for a wave-signal translating device comprising, a solid solution of silicon substantially free from impurities usually present in commercial silicon and a predetermined small amount of at least one of the two metals aluminium and beryllium, the ratio of the number of atoms of metal to the number of atoms of said silicon in the product having a value between limits of the order of ,4 0 and V50, said 1 silicon substantially free from impurities usually element having a surface oxidized under controlled oxidizing conditions and adapted to be contacted by a metallic contact element included in said translation device.

9. A silicon-crystal element for a wave-signal translating device comprising, a solid solution of silicon substantially free from impurities usually present in commercial silicon and a predetermined small amount of at least one of the two metals aluminium and beryllium, the ratio of the number of atoms of metal to the number of atoms of said silicon in the product having a value between limits of the order of and M50; said element having a surface oxidized in an atmosphere of oxygen for several hours at approximately.1050 centigrade and adapted to be contacted by a metallic contact element included in said translating device.

I 10. Asilicon-crystal element for a wave-signal translating device comprising, a solid solution of silicon substantially free from impurities usually present in commercial silicon and a predetermined small amount of at least one of the two metals aluminium and beryllium, the ratio of the number of atoms of metal to the number of atoms of said silicon in the product having a value between limits of the order of V and 1/5(), said element having a surface oxidized in air for ap-- proximately two hours at approximately 1050 centigrade and adapted to be contacted by a metallic contact element included in said translating device.

11. A silicon-crystal element for a wave-signal translating device comprising, a solid solution of silicon substantially free from impurities usually present in commercial silicon and a predetermined small amount of at least one of the two metals aluminium and beryllium, the ratio of the number of atoms of metal to the number of atoms of said silicon in the product having a value between limits of the order of and V50, said element having a polished, oxidized and hydrofluoric acid treated surface adapted to be contacted by'a metallic contact element included in said translating device.

12. A silicon-crystal element for a wave-signal translating device comprising, a solid solution of silicon substantially free from impurities usually present in commercial silicon and a predetermined small amount of at least one of the two metals aluminium and beryllium, the ratio of the number of atoms of metal to the number of atoms of said silicon in the product having a value between limits of the order of and said element having a surface initially oxide coated but so treated with hydrofluoric acid that the thickness of said oxide coating has been at least partially reduced.

13. A silicon-crystal element for a wave-signal translating device comprising, a solid solution of \present in commercial silicon and a predetermined small amount of at least one of the two metals aluminium and beryllium, the ratio of the of said silicon in the product having a value between limits of the order of and ,'said element having a first surface portion adapted to be contacted by a metallic contact element included in said translating device and having a second and metallized surface portion adapted to be electrically connected to a second metallic element of said device.

14. An electrical-crystal contact device comprising, a semiconducting crystal contact element comprised of silicon which contains a determinate small quantity of at least one of the two metals aluminium and beryllium in solid solution but which is otherwise substantially free from metallic impurities such as are usual in commercial silicon, the ratio of the'number of atoms of metal to the number of atoms of said siliconin the product having a value between limits of the order of and /50, and a metallic contact element co-operating with a polished and oxidized surface of said crystal element.

15. An electrical-crystal contact device com- 7 prising, a semiconducting crystal contact element comprised of silicon which contains an addition 7 of at least one of the two metals aluminium and beryllium in solid solution in the proportion of one atom of the additive metal to not less than 50 atoms of silicon but which is otherwise substantially free from metallic impurities such as are usual in commercial silicon, a metal member to which a copper-coated surface of said crystal element is soldered, and a metallic contact element pressed against a polished and oxidized surface of said crystal element.

16. An electrical-crystal contact device comprising, a semiconducting crystal contact element comprised of silicon which contains in solid solution a quantity of additive metal such that the ratio of the number of atoms of said additive metal to the number of atoms of silicon in the product is not greater than but which is otherwise substantially free from the metallic impurities usual in commercial silicon, and a metal* 'Eaecutria: the Estate of Stanley Vaughan Williams, Deceased.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,571,020 McGowan Jan. 26, 1926 2,402,582 Scarf June 25, 1946 2,402,839 Ohl June 25, 1946 2,419,561 Jones et a1. Apr. 29, 1947 

1. A SILICON-CRYSTAL ELEMENT FOR A WAVE-SIGNAL TRANSLATING DEVICE COMPRISING, A SOLID SOLUTION OF SILICON SUBSTANTIALLY FREE FROM IMPURITIES USUALLY PRESENT IN COMMERCIAL SILICON AND A PREDETERMINED SMALL AMOUNT OF AT LEAST ONE OF THE TWO METALS ALUMINUM AND BERYLLIUM, THE RATIO OF THE NUMBER OF ATOMS OF METAL TO THE NUMBER OF ATOMS OF SAID SILICON IN THE PRODUCT HAVING A VALUE OF THE ORDER OF 1/400. 